The present invention relates to a method for producing acetylated pyroxylin (or, in another parlance, cellulose acetate nitrate) used as starting materials for explosives, paints, etc. More particularly, the present invention provides a fine fiber form of acetylated pyroxylin having improved heat resistance and stability.
Acetylated pyroxylin, in which heat resistance is imparted to pyroxylin (or, in another parlance, nitrocellulose), is obtained by dissolving pyroxylin in a suitable organic solvent for an actylation reaction with anhydrous acetic acid, etc. in the presence of a suitable catalyst, and precipitating the reaction product in water or an alcohol (see JP-A""s 56-82849 and 8-277301).
One problem with this reaction for forming an acetic acid ester with respect to a residual hydroxyl group in pyroxylin is that the efficiency of using the starting anhydrous acetic acid is low. This is because any fast progress of the reaction is unachievable unless the anhydrous acetic acid is used in an amount that is 2 to 20 times as large as the theoretically required amount. An unreacted extra portion of the anhydrous acetic acid then reacts with a water or alcohol precipitation agent at the next precipitation step, and so changes to acetic acid or an acetic acid ester such as ethyl acetate, rendering it impossible to recover the anhydrous acetic acid as such. Another problem is the need of using a costly high-speed agitator at the precipitation step. Yet another problem is that the powder particle size of acetylated pyroxylin varies due to precipitation, causing the reaction solution component to be included in particles having a large diameter. This then appears in the form of impurities which result in unstable quality.
In view of the aforesaid problems, an object of the invention is to provide a method capable of recovering an extra portion of anhydrous acetic acid taking no part in a reaction with pyroxylin. Another object of the invention is to provide an acetylated pyroxylin product having improved heat resistance and stability by dispensing with any precipitation step and thereby simplifying production installations and eliminating an unstable quality factor due to product particle size variations.
As a result of study after study, the inventors have now found that pyroxylin is first subjected to an acetylation reaction while the pyroxylin is dispersed in a sort of dispersion medium and kept in a solid phase state, and acetylated pyroxylin is then separated from a reaction solution by means of filtration, so that an extra portion of the anhydrous acetic acid taking no part in the reaction can be immediately recovered and, hence, any precipitation step can be dispensed with to eliminate an unstable product quality factor due to product particle size variations.
Thus, the present invention provides a method for producing acetylated pyroxylin by action of an acetylating agent and a catalyst on pyroxylin, characterized by dispersing said pyroxylin in a dispersion medium to subject said pyroxylin to an acetylation reaction while said pyroxylin is kept in a solid phase state, and separating acetylated pyroxylin from a reaction solution by means of filtration.
The present invention also provides a fine fiber form of acetylated pyroxylin having improved heat resistance and stability.
According to one aspect, the present invention is embodied as follows.
(1) A method for producing acetylated pyroxylin by action of an acetylating agent and a catalyst on pyroxylin, characterized by dispersing said pyroxylin in a dispersion medium to subject said pyroxylin to an acetylation reaction while said pyroxylin is kept in a solid phase state, thereby obtaining crude acetylated pyroxylin, separating said crude actylated pyroxylin from a reaction solution by means of filtration, and washing said crude acetylated pyroxylin upon separation.
(2) The actylated pyroxylin production method according to (1), characterized in that said acetylating agent is anhydrous acetic acid, and said dispersion medium is an organic solvent which has a boiling point between 40xc2x0 C. and 200xc2x0 C. at normal pressure, in which said pyroxylin and said acetylated pyroxylin are insoluble and which is unreactive with said anhydrous acetic acid.
(3) The acetylated pyroxylin production method according to (2), characterized in that said dispersion medium comprises an aromatic hydrocarbon selected from the group consisting of aromatic hydrocarbons having 6 to 10 carbon atoms or a mixture of two or more such aromatic hydrocarbons.
(4) The acetylated pyroxylin production method according to any one of (1) to (3), characterized in that said catalyst is selected from the group consisting of p-toluenesulfonic acid and perchloric acid.
(5) The acetylated pyroxylin production method according to any one of (1) to (4), characterized in that a washing agent used for washing said crude acetylated pyroxylin is water and/or an organic solvent in which said acetylated pyroxylin is insoluble and which has a boiling point between 40xc2x0 C. and 200xc2x0 C. at normal pressure.
(6) The acetylated pyroxylin production method according to (5), characterized in that said organic solvent used as said washing agent is an alcohol having 1 to 4 carbon atoms.
(7) The acetylated pyroxylin production method according to (5), characterized in that a compound or compounds of an alkali metal and/or an alkaline earth metal are added into said water used as said washing agent.
(8) The acetylated pyroxylin production method according to any one of (1) to (7), characterized in that a process of washing said crude acetylated pyroxylin includes a step of holding said crude acetylated pyroxylin in said washing agent at a temperature of 60xc2x0 C. to 105xc2x0 C.
(9) An acetylated pyroxylin characterized in that fine fibers having a fiber diameter of 0.01 mm to 0.05 mm and a fiber length of at least 0.2 mm account for at least 90% by weight of all fibers, and characterized by showing a heat generation peak of at least 207xc2x0 C. upon differential thermal analysis as performed in an argon stream at a heating rate of 10xc2x0 C./min and a sample amount of 1 mg.
(10) The acetylated pyroxylin according to (9), characterized in that fine fibers having a fiber diameter of 0.015 mm to 0.04 mm and a fiber length of at least 0.4 mm account for 90% by weight of all fibers.
According to another aspect, the present invention is embodied as follows.
(11) The acetylated pyroxylin production method according to any one of (1) to (3), characterized in that said catalyst is at least one acid from the group consisting of p-toluenesulfonic acid and perchloric acid.
(12) The acetylated pyroxylin production method according to any one of (1) to (4), characterized in that a washing agent used for washing said crude acetylated pyroxylin is water and/or an organic solvent in which said acetylated pyroxylin is insoluble and which has a boiling point of 40xc2x0 C. to 200xc2x0 C. at normal pressure.
(13) The acetylated pyroxylin production method according to any one of (1) to (4), characterized in that a washing agent used for washing said crude acetylated pyroxylin is water and/or an organic solvent in which said acetylated pyroxylin is insoluble and which has a boiling point of 40xc2x0 C. to 200xc2x0 C. at normal pressure.
(14) The acetylated pyroxylin production method according to (5), characterized in that a compound or compounds of an alkali metal and/or an alkaline earth metal are added to said water used as said washing agent.
(15) The acetylated pyroxylin production method according to (5), characterized in that a compound or compounds of an alkali metal and/or an alkaline earth metal are added to said water used as said washing agent.
(16) An acetylated pyroxylin obtained by the production method according to any one of (1) to (8) and (11) to (15).
(17) The acetylated pyroxylin according to (16), characterized by being an acetylated pyroxylin as recited in (9) or (10).
(18) The production method according to any one of (1) to (8) and (11) to (15), characterized in that an acetylated pyroxylin as recited in (9) or (10) is obtained.
According to yet another aspect, the present invention is embodied as follows.
(19) The production method according to (1), characterized in that anhydrous acetic acid is used as said acetylating agent.
(20) The production method according to any one of (1) to (8), (11) to (15) and (19), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of 3 to 15 moles per mole of a hydroxyl group in the pyroxylin to be acetylated.
(22) The production method according to any one of (1) to (8), (11) to (15) and (19) to (21), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of up to 20 parts by weight per 100 parts by weight of said dispersion medium.
(23) The production method according to any one of (1) to (8), (11) to (15) and (19) to (21), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of up to 15 parts by weight per 100 parts by weight of said dispersion medium.
(24) The production method according to any one of (1) to (8), (11) to (15) and (19) to (20), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of 2 to 20 moles per mole of a hydroxyl group in the pyroxylin to be acetylated and up to 20 parts by weight per 100 parts by weight of said dispersion medium.
(25) The production method according to any one of (1) to (8), (11) to (15), (19) and (21), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of 3 to 15 moles per mole of a hydroxyl group in the pyroxylin to be acetylated and up to 20 parts by weight per 100 parts by weight of said dispersion medium.
(26) The production method according to any one of (1) to (8), (11) to (15) and (19) to (20), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of 2 to 20 moles per mole of a hydroxyl group in the pyroxylin to be acetylated and up to 15 parts by weight per 100 parts by weight of said dispersion medium.
(27) The production method according to any one of (1) to (8), (11) to (15), (19) and (21), characterized in that anhydrous acetic acid is used as said acetylating agent, said anhydrous acetic acid being used in an amount of 3 to 15 moles per mole of a hydroxyl group in the pyroxylin to be acetylated and up to 15 parts by weight per 100 parts by weight of said dispersion medium.
(28) An acetylated pyroxylin obtained by the production method according to any one of (19) to (27).
(29) The acetylated pyroxylin according to (28), characterized by being an acetylated pyroxylin as recited in (9) or (10).
(30) The production method according to any one of (19) to (27), characterized in that an acetylated pyroxylin as recited in (9) or (10) is obtained.
According to the present invention, there is provided a method for producing acetylated pyroxylin by the reaction of pyroxylin with anhydrous acetic acid, which enables a portion of the anhydrous acetic acid taking no part in the reaction to be recovered after the reaction. With this method, production installations can be simplified, and stable quality can be achieved as well. The present invention also provides acetylated pyroxylin having improved heat resistance and stability.
According to the present invention, pyroxylin is first subjected to an acetylation reaction while the pyroxylin is dispersed in a dispersion medium such as toluene and kept in a solid phase (fine fiber form) state. Then, acetylated pyroxylin is separated from a reaction solution by means of filtration, so that an extra portion of the anhydrous acetic acid taking no part in the acetylation reaction can be immediately recovered. This can dispense with the precipitation process needed for a prior art solution reaction method, and so can eliminate an unstable quality factor due to product particle size variations. In addition, the process of washing the post-reaction crude acetylated pyroxylin is designed in such a manner that the crude acetylated pyroxylin is washed at a high temperature of 60 to 105xc2x0 C. and with a stabilizer-containing washing agent. It is thus possible to provide acetylated pyroxylin having improved heat resistance and stability.
Anhydrous acetic acid is usually used as the acetylating agent. Referring the first limitation to the amount of anhydrous acetic acid used, it is required that anhydrous acetic acid be used in an amount of 2 to 20 moles, and preferably 3 to 15 moles per mole of a hydroxyl group in the pyroxylin to be acetylated. At less than 2 moles, the acetylation reaction does hardly proceed. On the other hand, the addition of anhydrous acetic acid in an amount of greater than 20 moles is insignificant. Referring to the second limitation to the amount of anhydrous acetic acid used, it is required that anhydrous acetic acid be used in an amount of up to 20 parts by weight, and preferably up to 15 parts by weight per 100 parts by weight of the dispersion medium.
At greater than 20 parts by weight, pyroxylin may possibly be dissolved in the reaction solution, i.e., a medium composed mainly of a solution mixture of the dispersion medium and anhydrous acetic acid, because the anhydrous acetic acid is a good solvent for pyroxylin. This may make it impossible to conduct the reaction of pyroxylin with anhydrous acetic acid while the pyroxylin is kept in a solid phase state. In the present invention, the first and second limitations to the amount of anhydrous acetic acid should be satisfied at the same time.
For the dispersion medium, it is required to use a medium in which both pyroxylin and acetylated pyroxylin are insoluble. For such a medium, it is preferable to use an organic solvent having a boiling point of 40xc2x0 C. to 200xc2x0 C., and especially 50xc2x0 C. to 150xc2x0 C. at normal pressure. Examples of such an organic solvent are aromatic hydrocarbons having 6 to 10 carbon atoms, as represented by benzene, toluene, and xylene. For the dispersion medium, these organic solvents may be used alone or in admixture of two or more. Preferably, the amount of the dispersion medium should be 10 to 80 times, and especially 15 to 40 times as large as the weight of pyroxylin. At less than 10 times non-uniform reaction may occur due to insufficient agitation of slurry, and at greater than 80 times production efficiency becomes worse although the present invention may be somehow carried out.
For the catalyst, general acetylating catalysts such as sulfuric acid, pyridine and various amines may be used. However, it is preferable to use at least one selected from the group consisting of p-toluenesulfonic acid and perchloric acid. Of these catalysts, perchloric acid is most preferred because it is effective in a small amount and can yield acetylated pyroxylin having high stability. These catalyst may be used in admixture. The acetylating catalyst should preferably be used in a net amount of 0.1 to 5.0 parts by weight, especially 0.3 to 2.0 parts by weight for perchloric acid and 5 to 60 parts by weight, especially 10 to 50 parts by weight for p-toluenesulfonic acid, as calculated per 100 parts by weight of the starting pyroxylin. Too little a catalyst makes the acetylation reaction less likely to occur, and too much presents some problems such as possible denitration of pyroxylin and a waste of catalyst.
The starting pyroxylin material used in the present invention should comprise fine fibers having a fiber diameter of 0.01 mm to 0.05 mm, and preferably 0.15 mm to 0.04 mm, and should be in such a form that they can be dispersed in the dispersion medium. This is because the pyroxylin is subjected to the acetylation reaction while it is kept in a solid state. Usually, pyroxylin is available in the form of an aggregate comprising fine fibers having the aforesaid fiber diameter and a fiber length of 0.5 mm to 2.0 mm. In the present invention, however, the fiber aggregate should be disintegrated into individual fibers so that they can be well stirred in the dispersion medium. If the fibers in such a state drift in the dispersion medium, the solution can then have a uniform composition everywhere so that the acetylation reaction can occur satisfactorily. With the solution composition according to the present invention, the fibers are swollen to a certain extent with the result that the substance needed for the reaction penetrates into the fibers, so that the acetylation reaction can occur not only on the surfaces of the fibers but also in the interiors of the fibers. In addition, it is easy to remove impurities from within the fibers by means of extraction and washing in the post-reaction washing process, because the fibers have a small diameter as already mentioned. It is thus possible to obtain acetylated pyroxylin having improved heat resistance and stability.
The degree of acetylation of the acetylated pyroxylin according to the present invention may be varied by a choice of the degree of nitration of the starting pyroxylin. In this regard, however, it is noted that the degree of nitration of acetylated pyroxylin may become lower than that of the starting pyroxylin due to denitration of the starting pyroxylin during the production process. Accordingly, the starting pyroxylin should be selected while such a case is taken into account. In the present invention, all pyroxylin materials with known degrees of nitration are available. However, it is preferable to use as the starting material a pyroxylin having a degree of nitration of 0.5 to 2.6, and preferably 1.0 to 2.5. At less than 0.5 any significant feature can hardly be imparted to the resultant acetylated pyroxylin because the performance difference between the acetylated pyroxylin and the starting pyroxylin is small. At greater than 2.6, too, there is little or no characteristic difference between the resultant acetylated pyroxylin and the starting pyroxylin. The degree of nitration of pyroxylin may be determined as by conducting elemental analysis to find the amount of nitrogen.
Basically, the degree of acetylation of the acetylated pyroxylin according to the present invention becomes lower than the amount of the residual hydroxyl group in the starting pyroxylin. In some cases, however, this degree of acetylation becomes higher than the amount of the residual hydroxyl group although depending on the aforesaid degree of denitration. The degree of acetylation may be controlled by the reaction temperature and time as well as the amount of the catalyst and anhydrous acetic acid used. The degree of acetylation should be in the range of 0.3 to 2.0. At less than 0.3, the heat resistance of the acetylated pyroxylin is as low as that of the starting pyroxylin; the effect by acetylation is slender. At greater than 2.0, there is little or no characteristic difference between the acetylated pyroxylin and the starting pyroxylin.
The reaction temperature should preferably be in the range of 25xc2x0 C. to 50xc2x0 C., and especially 30xc2x0 C. to 45xc2x0 C., and the reaction time should preferably be selected from the range of 0.5 hours to 5 hours, and especially 1 hour to 4 hours. Within these temperature and time ranges, the degree of acetylation may be controlled. When there is a deviation from these temperature and time ranges, i.e., when the reaction temperature is too low or the reaction time is too short, however, any acetylation reaction does not substantially occur. When the reaction temperature is too high or the reaction time is too long, on the other hand, denitration is likely to occur or other problem may arise.
After the acetylation reaction of the starting pyroxylin, it is required in the present invention that crude acetylated pyroxylin be separated from the reaction solution by means of filtration. It is then possible to recover anhydrous acetic acid, the organic solvent used as the dispersion medium, and reaction by-products such as acetic acid from the filtrate by means of generally used distillation processes, etc. In the present disclosure, the term xe2x80x9ccrude acetylated pyroxylinxe2x80x9d is used to stress that the acetylated pyroxylin is in a green or unpurified state.
The crude acetylated pyroxylin obtained after filtration is of low stability, partly because the reaction solution does not only deposit on the surface of the pyroxylin but also penetrates into the pyroxylin, and partly because the catalyst bonds to the interior of the pyroxylin. It is thus required that the crude acetylated pyroxylin be repeatedly washed and filtered with water and/or the washing agent. For washing, water is primarily used because of its inexpensiveness. However, it is acceptable to use other washing solvent optionally with water. For the washing agent in this case, use may be made of an organic solvent in which acetylated pyroxylin is insoluble. For instance, an aromatic hydrocarbon having 6 to 10 carbon atoms or a mixture of two or more such hydrocarbons used as the dispersion medium may be used. Particularly preferred examples of the washing agent are alcohols having 1 to 4 carbon atoms, for instance, methanol, ethanol, n-propanol, isopropanol, n-butanol, and isobutanol.
Preferably, the washing process should include a thermal treatment step of holding the crude acetylated pyroxylin in water and/or the washing agent at a temperature of 60xc2x0 C. to 105xc2x0 C., because the stability of the acetylated pyroxylin becomes much better. Preferably in this case, the thermal treatment temperature should be in the range of 60xc2x0 C. to 105xc2x0 C. and the thermal treatment time should be in the range of 1 hour to 6 hours. At lower than 60xc2x0 C. the effect on stability improvements becomes insufficient, and at higher than 105xc2x0 C. there is a possibility that the acetylated pyroxylin may be slowly disintegrated. The thermal treatment may be repeatedly conducted with varying holding media. Usually, it is preferable that the thermal treatment be carried out in boiling water. In addition to, before or after that thermal treatment, however, it is acceptable to carry out another thermal treatment at a suitable temperature between 60xc2x0 C. and 105xc2x0 C. in an alcohol having 1 to 4 carbon atoms, for instance, methanol, ethanol, n-propanol, isopropanol, n-butanol, and isobutanol, of which isopropanol is most preferred.
As a stabilizer for the acetylated pyroxylin, it is preferable to add a compound or compounds of an alkali metal and/or an alkaline earth metal into the water acting as the washing agent. For the compounds of the alkali metal and alkaline earth metal, it is acceptable to use hydroxides of metals such as sodium, potassium, calcium, magnesium and strontium, or salts of these metals with weak acids represented by carbonic acid, acetic acid, etc. Exemplary compounds are sodium carbonate, sodium acetate, calcium hydroxide, and calcium acetate. In use, one or two or more of these compounds are first dissolved in water at a concentration of about 10 ppm to 5,000 ppm, and preferably about 50 ppm to 1,000 ppm. The washing process and/or the thermal treatment process should preferably be carried out, using the thus obtained solution. At a concentration of less than 10 ppm, stability may possibly become insufficient. At greater than 5,000 ppm, the effect for justifying this amount cannot be obtained although there is no grave problem.
In the present invention, the stabilizer-containing water is not necessarily used throughout the washing process and/or the thermal treatment process. In other words, it is usually acceptable to use the water somewhere in the washing process and/or the thermal treatment process. In this connection, it is noted that the stabilizer may be added into the washing solvent, especially an alcohol having 1 to 4 carbon atoms.
The acetylated pyroxylin produced according to the present invention explained above is excellent in terms of heat resistance and stability, and so can be used as raw materials for explosives, paints, etc.